Service engine soon light

According to the previous owners inspection print out the MAF needs to be replaced. I bought some MAF cleaner and sprayed it. I also changed the air filter. It was a wreck. I expected the service engine soon light to go off but it didn't. I found out how to read codes from the ac controls and it isn't showing any except on the hvac trouble codes display. There it shows 126 which is a history code right?

Based on the state of the air filter I'm going to do a full tune up soon and also replace the MAF sensor.

Is there a way to reset or shut off the service engine soon light or does it go off by itself?

press the 'off' button while the code is displayed in the hvac,.

! in front means its a history code,. 26 dtc is an evap fault,.

If the check engine light is on,. you have an ongoing dtc,.

So it doesn't turn off by itself after a while?

And would a code reader give me more info than the read out on the dash? Forgive my possible dumb questions. I've never owner a car with a computer.

I have the same question about how to reset the Service Engine Soon idiot message. Anybody know what the trick is? Thanks.

Like Phantom309 said, press the "off" button while the code is displayed to reset it. If it comes back, then the problem is ongoing.

A code reader won't give you a lot more info than the climate control display. However, a scan tool will give you a heck of a lot more. Most significantly, a proper scan tool will allow you to monitor the actual live data streams.

The problem with trouble codes is that they are often misleading. For example, an "O2 sensor" code may set if there is a problem with the O2 sensor, but the same code may set if there is a problem with the fuel pressure or the wiring, or a ground point. The code is a starting point for diagnosis, and the Factory Service Manual comes with charts of all the things to be checked -- and many times, the checking requires (or is at least, is simplified by) sampling the live data that the ECU/PCM is seeing.

So will a scan tool allow me to shut off the service engine light? I can't seem to find any info on shutting off that light. Like I said the ac display isn't showing any codes so I'm not really sure what the issue is here. Maybe I should just buy a new MAF sensor and see if that shuts off the light.

I had the exact problem with my 96 Roady, and did replace the MAF sensor. It was under a $100, and took about 10 minutes to do. However, the light still goes on occasionally (usually when it is cold) but goes out by itself in a day or so. I've given up. The car runs/drives fine, it doesn't seem to affect the gas mileage, so I am just accepting it is some quirk of an otherwise good running 19 year old car.

Mattb wrote:

So will a scan tool allow me to shut off the service engine light? I can't seem to find any info on shutting off that light. Like I said the ac display isn't showing any codes so I'm not really sure what the issue is here. Maybe I should just buy a new MAF sensor and see if that shuts off the light.

A scan tool will reset any DTC codes. If the code does not return right away, then the MIL light will go out. Unless of course, there is a problem with the MIL light circuit itself, but then, that should set a DTC 11 code. AFAIK, only a Tech2 can turn the light on and off at will, and then only so long as the Tech2 is connected to the diagnostic port.

So first thing I would do is double check that you have read the codes in the CC panel correctly. Your car is a 94, 95 or 96, right?
1. Start the car
2. Press and hold the "temp up" and "OFF" buttons simultaneously until the whole display lights up like a Christmas tree.
3. Release both buttons. Display should show -00, indicating the system is in diagnostic mode
4. Press the "middle" Button.

If there are no DTCs set, the display should change from "-00" to "00". If there are DTCs set, then the display should scroll through them in sequence, unless there is a problem with the serial communications bus (see discussionion of HVAC code 26, below).

There are no conditions that I am aware of that will light up the MIL but not set a DTC. There are a few conditions that will set a code but not light the MIL. There are a very few that will clear the MIL immediately while driving if the problem corrects itself. There are a few more that will reset the MIL automatically the next time the key is cycled if the problem corrects itself. Most will keep the MIL lit until the DTCs are cleared.

Check to see if the system is working by unplugging the MAF sensor and see if you get "48" in place of "00". When "48" is displayed, press "off". Code should reset to "00".

(Another option would be to visit an auto parts store that offers free code scanning services.)

As Phantom309 pointed out, an engine DTC 26 is related to the EVAP canister purge solenoid. However, if you are decoding the CC display properly, you are seeing an HVAC DTC 26, which is a serial communications error code and should not set the MIL. In fact, that code is often a real "red herring" as it can accidentally be set just by using a scan tool to query other (IE: non-HVAC) systems. (The scan tool monopolizes the serial interface and the HVAC 'puter gets a bit upset when it can't talk to it's buddies.) This could have happened when the PO had the car scanned. If there is a current problem with the serial bus, then the code should read "26", not "126".

Finally, these cars are not that sophisticated. You can reset all codes by disconnecting the battery for 30 seconds. Of course, that will also reset the time on the clock, the learned fuel tables, the transmission adaptive shift tables, etc. etc.

A word of warning: if your security lamp is lit all the time, don't disconnect the battery until you have cured any PASS Key issue. You may not be able to restart the car again. If the PASS key circuit failed while the car was running, the anti-theft system will disable itself and allow the car to be restarted despite the failure. If the system is reset with the engine off (eg: disconnecting the battery) and subsequently fails to detect the correct key has been inserted, it will assume someone is trying to steal the car and prevent the engine from starting.

Thanks for that. I'll try unplugging the MAF and see if it displays 48.

For -00 (PCM diagnostic mode) and -01 (CCM diagnostic mode) when I press the "middle" button the display shows -- not 00. Is that indicative of something?

I disconnected the MAF and the display still reads "--". No code just two dashes. What does this mean? Should I try disconnecting the battery?

I think it's indicative of a communications failure between the two modules. Interesting. That certainly would explain why you have no PCM DTCs -- even when disconnecting the MAF.

What year is the car?

At this point, I would say you are going to need a code reader or scan tool to find the reason for your MIL. Unless there is a break in the line between the diagnostic link connector and the PCM.

Do you get anything other than dashes when looking at the ABS or Airbag modules (ie: 3 and 4)? What about if you try to read a specific datastream like RPM (6)? That should show a live feed that fluctuates as you rev the engine.

It's a 96. -02, -03, -04 all show 00. -02 used to say 126 but I cleared that. The live stream stuff all works properly.

Well if the live stream of PCM data is working, then there's communication between the climate control and the PCM over the serial bus. The mystery is then why doesn't the PCM supply the DTC codes.

I wonder if it's because it's a 96. 1996 models are OBDII. They monitor more stuff, are programmed with a lot of additional codes, and output 4 digit codes to a scan tool or code reader. I've never heard that they won't output codes to the CC display, but I suppose it's possible.

The good news is that they are OBDII, meaning any code reader can retrieve the basic codes. You don't have to worry about finding someone who still has the old OBDI interface. If you were closer (I'm in Ontario, Canada), I'd happily hook it up to my Tech2. Maybe there's someone on this forum who's nearby with the appropriate tool?

That's interesting. I hope you're right about it just not reading the info. I wonder if somebody on here has a 96 and can check if theirs displays codes on -00 and -01.

I'm going to get a scan tool ASAP.

Unplugging the battery for 30 seconds should reset the codes though right? If like to see if cleaning the MAF worked or if I need to buy a new one.

I unplugged the battery and the light went off. It also didn't come back on while driving. So, great. I guess cleaning the MAF worked.

In the diagnostics mode the display still shows "--" for both -00 and -01. I'm not going to worry about that too much. Going to buy scan tool today.

Ha. It just came back on.

Here's a silly question: where do you plug the scan tool in? Is it under the hood or in the car or both?

Don't rush out and buy the first scan tool you see. Do a bit of research first, and consider your needs. I have 5 scan tool solutions. Two are dedicated tools, three are software based. I use three of them regularly.

The ultimate scan tool for GMs up to about 2013 is the Tech2 (and you can get a Chinese clone for around $300). No other scan tool can do all the things a Tech2 can do -- on a GM. The Tech2 is pretty much useless on a Ford. There are a number of aftermarket tools out there, with varying levels of support and usability on various makes. So consider your fleet and probable future fleet then look for the tool that will be most useful for you.

As for your returning MIL, that kind of delay usually indicates the issue is more than just a broken wire or bad sensor. For example, if the code for, say, Bank1 Sensor 2, O2 sensor comes up, the problem may actually be the cat converter. Or if the MAF code comes up, the problem may actually be an air pressure sensor (the PCM calculates a rough fuel/air mix based on TP, air pressure, temperature, etc. and if the MAF is not in the same ballpark, the PCM assumes the MAF is wrong rather than one of the other sensors). That's where having an actual scan tool that can read the actual raw data like MAF output or manifold pressures are a great help in diagnosing the real cause of a problem.

The diagnostic port is under the dash, just to the right of the steering column. OBDII requires it be within so many inches of the driver's seat so you don't see them under the hood anymore. That was usually old Fords and Chryslers anyway -- GM had them under the dash from the start.

Later tonight I'll post what the codes were from the inspection place. He gave me the printout with the DTC numbers. I remember there were two different MAF codes and one secondary air injection system code.

Also I should mention, the air filter was totally shot. Really dirty and kind of rusty. I changed it. Maybe it was so messed up that it screwed up the MAF.

The codes the inspection station found were: P0100 mass air flow circuit malfunction, P0103 mass air flow circuit high input and P0412 secondary air injection switching valve A circuit malfunction.

It appears the guy went twice and failed both times.

It also reported MIL commanded fail and KOER fail.

That's malfunction indicator lamp and key on engine running but I'm not sure what it really means.

I looked at the fuse box under the hood and noticed the air pump relay was missing. The fuse was good. Maybe this is part of the problem with the P0412 code. I'll get a relay tomorrow and put it in and see what happens. Here's a photo of it. [You must be registered and logged in to see this image.]

It should be under the two smaller relays. The plastic looks cleaner there so it must have been removed recently. I wonder why it was removed.

With some research I figured it out. Air pump delete procedure. It was only half done though. They removed the relay but didn't plug the hose at the air cleaner and no special sticker. I can only assume they didn't reprogram the pcm either. This explains all three codes.

So what now? Replace the relay? Finish the job? Is the MAF screwed up because of this? I'm inclined to finish the job but I'll have to reprogram the pcm to keep the light from turning on.

Ah. Sounds like the old "water-in-the-air-pump" issue. Two of them, anyway.

Read this TSB:  [You must be registered and logged in to see this link.]

To program the updated calibration, you will need access to a Tech2, or have the PCM reprogrammed by a third party (eg: pcmforless, etc.)

The third, P0103, is exactly what I was talking about earlier. The PCM has calculated the speed/density based on MAP, RPM and IAT sensor inputs. Then it compares the MAF frequency to the expected value base on the speed/density calculation. Based on that, the PCM either: found the MAF frequency to be consistently higher than expected OR the PCM noted that the MAF frequency is not changing, stuck high.

The only scan tool that I know of that will read the actual MAF frequency value is the Tech2, however any decent scan tool should be able to provide the equivalent MAF value that the PCM calculated from the frequency. You need to look at the data stream for the IAT and MAP sensors and see if they are reasonably close. (EG: engine off MAP should be around 101kpa, at idle +/- 25kpa, engine cold Intake Air Temperature should be close to the actual ambient temperature.)  You also need to look at the MAF frequency (or MAF value) to verify whether it is fluctuating with engine RPM as expected.

Edit: I see you posted while I was typing. I don't think P103 has anything to do with the air pump. You could try replacing the relay, but odds are you will find that doesn't work for long either (that's why GM came up with the AP disable TSB). I think you are likely better off completing the disable.

unplug the air pump,. then install the relay

If you are going to go that route, you could replace the relay with an 82 ohm, 3 watt resistor across terminals 85 and 86. Wouldn't even need to unplug the AIR pump, the resistor would fool the PCM into thinking the relay is in place.

Edit: to figure out which ones are 85 and 86, unplug one of the other relays and look for the numbers on the bottom.

That's a good idea guys. Thanks.

What exactly does the re-tune do? Does it only tell the pcm not to send a code in the absence of the relay?

Actually, after googling P0412, I don't think that will work after all. It seems the PCM is also monitoring the supply voltage to the AIR pump itself. If the pump ain't there, the supply voltage won't drop and P0412 will set. The reprogramming procedure deletes the various checks, including not only voltage checks at the pump and relay but also looking for a change in the O2 sensor when the AIR is commanded on.

Gotcha. Well, I can live with the light for a while.

Regarding the MAF, what are my options? I cleaned it but don't have a scanner to see if the code is still present. The air filter was rusty, had holes, looked like it was wet at some point, had cracks in the rubber seal; do you think that could have affected the MAF related P0103 code?

I think there is a strong possibility that the MAF is bad. Personally, I would check the datastream with a scan tool before replacing it, but then, I have a scan tool. You could at least check the integrity of the wire and power supply with an ohm meter -- that won't tell you the MAF is good, but could confirm it is bad (broken sensor wire) or that it's not getting power. Do you have a FSM? It outlines the tests.

I don't have a service manual yet. That and a scan tool are at the top of my list. Looks like the tech 2 is going to be the one to buy.

Or not, I didn't realize they were so expensive. I'll find something a little more affordable. Do you have any experience with the iPhone app scan tools?

You might want to look at this thread:

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Oh, and no, I have no experience with iPhone anything. My software based "scan tools" run on PC laptops and Palm Pilots. I have AutoEnginuity (the best of that bunch, but gets expensive if you have a lot of different makes to deal with) and OBDScan (seems to have been abandoned by the developer -- no updates in years.) The third is a specialty item for old Mitsubishi vehicles like the Dodge Stealth.

In hardware, I have the Tech2 (clone, as above) and the OTC4000E with a wealth of cartridges. Unfortunately, they stopped supporting that tool in 2001, but it's probably the best (together with the Snap-On equivalent) generic scan tool for cars of that era. The interface is actually nicer than the Tech2, but the Tech2 does some specialty things that no generic can do, and the FSM is written with the assumption a Tech2 is available.

Thanks for posting that. I immediately thought the Chinese clones were going to be garbage.

Just to be clear on one thing, you need to download the Tech 2 software from a windows computer with the old rs232 serial port on it?

You are much more knowledgable than I am in this department, what do you think of this?

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It's a bluetooth scan tool that connects to an iPhone or Android via an app. It gets pretty good reviews. For my purposes I think it seems pretty good. Also a good price. I know it doesn't reprogram the PCM but I don't think I'd need that very often, other than right now obviously with the air pump thing. It clears codes though, which I will definitely need.

I've got my 3yr old grandson over and I'm trying to paint the shed, so I can't afford to leave them alone together long enough to investigate that tool in depth. The most important things to investigate is the depth of coverage of "enhanced codes" -- those are the ones over and above the minimum required common OBDII codes, and are manufacturer-specific. Companies either have to buy the data from the manufacturer or figure it out for themselves and so exactly what constitutes "enhanced coverage" varies wildly from tool to tool.

Second thing I would look at is the number of items that can be monitored simultaneously and the speed of update (sometimes called "frame rate"). If you are looking for an intermittent glitch, a slow frame rate might miss something. To a certain extent, this is dependent on the car's interface and older cars like ours will not be as fast as CANbus cars. However, I notice that my Tech2 is considerably faster than AutoEnginuity.

Third is recording capability. You want to be able to capture all data as you drive when diagnosing a problem, then review and graph at your leisure afterwards. You may also want to trigger the start of recording when a code is initially set -- including the 15 or 30 seconds before the code was triggered.

It seems pretty advanced, but when you have a chance take a look at it and let me know...if you don't mind.

Thanks!

Had a quick look at some on-line reviews. Looks better than a code reader, but limited in scope as far as enhanced systems.

Cons: A complaint that it read very few items on one particular car. It doesn't even pick up the VIN, which is an available function on most cars.

Early complaints about charges for additional vehicles are no longer applicable and everything is free (after the initial purchase price). Considering how much auto manufacturers charge for info about enhanced code features, I suspect this means there's very little ongoing development or updating, particularly with regards to enhanced data.

It apparently does record, but just dumps a comma delimited data file. You would have to manipulate that yourself to turn it into a meaningful display.

Engine codes only for the Roadmaster -- this will not help if you need to diagnose and repair Climate Control, Air bags, or Anti-lock systems. Probably no help with transmission issues either, but I'm not certain of that (same computer, but not mandatory OBDII)

No control output (eg: command EGR on, etc. Necessary for some troubleshooting procedures)

Pros: At $100, it's fairly cheap if you have an iPhone already.

They claim to be able to read at least some of the enhanced codes for GMs, but do not stipulate the specifics, leaving a huge grey area. But still, some is better than what you get with, say, AutoEnginuity until you shell out for the enhanced packages.

From the images, it appears to display at 8 items from the datastream at a time, which is pretty good.

From the sample log file I downloaded, it seems to be logging 18 points per second, which is a pretty good frame rate. (note: Only 6 sensors were being logged. I don't know if that's the limit or just what they happened to sample. I've had AutoEnginuity logging 30 or 40 sensors at a time.)

It does grab the OBDII self-test results which many jurisdictions are using in lieu of a smog test now. Handy if you have to deal with that stuff.

It does grab the freeze-frame data that the PCM stores with a trouble code, so you have an idea what the circumstances were at the moment the code was set.

Compact and wireless. You could keep that dongle in the car at all times -- heck, you could probably leave it plugged in at all times (although that's a drain on the battery). Assuming you have your phone with you, it might come in real handy if you have problems while on the road.

My overall impression? This is kind of like buying a screwdriver set from Walmart. It is better than the toy set you would get at a corner variety store, and the handles look pretty, but the shaft is a special alloy of steel and peanut butter that only works if the screw is not too tight. It will probably do for the average person. The set from Matco costs 10x more, but will last a lifetime and the only way it won't work is if the screw head snaps off.

But, bear in mind that I have never seen one of these in person. I could be completely wrong. And the price is comparable to ONE diagnostic session at your local garage, so it could pay for itself quickly. The sample log I downloaded DID log MAF data, although I have no idea what car that came from and therefore can't guarantee it will log it on a Roadmaster. But it is possible. They advertise "Free Returns", so if it doesn't do what you need, then send it back.

Mattb wrote:

Just to be clear on one thing, you need to download the Tech 2 software from a windows computer with the old rs232 serial port on it?

No, there are USB to Serial adapters, and certain ones will work. But a real Serial port will always work. Basic functions like downloading and graphing saved data files will work just fine like that.

A real physical parallel port (ie: "LPT1") is required for advanced features. The tool is not connected to the parallel port, but GM supplies dealers with a security dongle that plugs into the parallel port to prevent others from accessing the dealer only functions like reprogramming the PCM. But "a padlock only keeps out the honest man" as they say and there is a software crack floating around that can fool TIS2000 into believing the dongle is attached to the port. But the crack still requires the presence of a physical parallel port to work.

I'm not advocating it, I'm just saying it's out there.

most auto parts stores will read the code (s) for free,...

phantom 309 wrote:

most auto parts stores will read the code (s) for free,...

Which is, IMHO, the equivalent of the variety store screwdriver set.

Auto parts stores make a lot of money from people trying to fix things by swapping in parts, one after the other, often because they mistake a "code" for a diagnosis, fail to understand how the overall system works, and don't realize the multitude of causes that can be behind any particular DTC.

Thanks for your response about the scanner. I guess I'm just wondering if I need the kind of in depth analysis the tech 2 provides. I downloaded the app and entered my vin and it stated enhanced diagnostics supported. Where did you see that it supports engine codes only on the roadmaster?

At the bottom of the main page is a drop-down menu to select a vehicle. If I select a 96 Roadmaster, it reports that it can "Read/Clear Check Engine Codes".

However, if I try certain other random vehicles (eg: 2010 Dodge Grand Caravan), it reports
"Read/Clear Check Engine Codes
Read/Clear Enhanced Codes (e.g. ABS, Airbag, Transmission, etc.)"

I can only go by the fact that they draw a distinction in their website. As I said, I have no personal experience with this software. I cannot say what it will and won't do with any certainty. It may well be that they have added some functionality to their software and failed to update the webpage.

You may not need all a Tech2 can do, but you can't do any of the extra things a Tech2 can do without one. Assuming you could do those things -- you don't even have a FSM (yet). Realistically, what skill set are you bringing to the table? Are you likely to grow into the extra capabilities? Are you going to be servicing more than one pre-2013 GM vehicle over time?

If Tech2s were the same price as the clones, I'd say buy the Tech2. If there was no risk to buying a Chinese Tech2 clone, I'd go with the clone.

But the reality is that GM charges about $4,000 for their version and there is an element of risk to buying a Tech2 clone from China. You can mitigate the risk as I describe in that linked thread, but even if you get one that works out of the box, it could die 2 minutes after confirming the receipt and releasing the payment from escrow. Then you are at the mercy of some guy in China and his unenforceable warranty.

The Lemur is from a Canadian company with a well-established business presence (of course, they may just be one man, a garage, and a good website developer -- you can't really tell these days!) that have been around for a few years. They offer free returns. They have something to lose if their reputation is tarnished. I rate that as low-risk in case of failure or dissatisfaction.

So you are faced with a classic risk-management decision. Do you go with the low-risk sure thing in return for lesser capability, or accept higher risk for higher potential reward? I can't make that decision for you.

Good advice. I'll weigh my options and figure it out.

buickwagon wrote:

You may not need all a Tech2 but you can't do any of the extra things a Tech2 can do without one. Assuming you could do those things -- you don't even have a FSM (yet). Realistically, what skill set are you bringing to the table? Are you likely to grow into the extra capabilities? Are you going to be servicing more than one pre-2013 GM vehicle over time?

Sometimes in life,. all you need is the variety store screwdriver set,. to tighten up 1 screw,..

phantom 309 wrote:

Sometimes in life,. all you need is the variety store screwdriver set,. to tighten up 1 screw,..

Absolutely.

So now I'm only getting a P0412 code. I had them check it at autozone. Is my only option getting the PCM reprogrammed?

Mattb wrote:

So now I'm only getting a P0412 code. I had them check it at autozone. Is my only option getting the PCM reprogrammed?

put the relay back in,.  unplug the pump,.make sure the fuse is good,.

If the hood light works,. you're good to go,.. cel will go out,..